Process for producing carbon membranes

ABSTRACT

There is described a process for producing carbon membranes suitable for gas separation, comprising the steps of:
     a) coating a porous substrate with a solution of at least one organic polymer which can be converted to a carbon membrane by pyrolysis,   b) drying the polymer coating on the porous substrate by removing the solvent,   c) pyrolyzing the polymer coating on the porous substrate to form the carbon membrane suitable for gas separation,   

     it being possible to conduct any of steps a) to c) or the sequence of steps a) to c) more than once, and the pyrolysis in step c) being effected at a temperature higher than the baking temperature of the porous substrate.

The invention relates to processes for producing carbon membranes suitable for gas separation, and to carbon membranes obtainable by the process.

The use of carbon membranes for gas separation is known per se. This exploits the interlayer spacing of graphite, which at 3.35 Å is within the order of magnitude of the size of small gas molecules.

The carbon synthesized by the pyrolysis of organic materials is referred to as paracrystalline carbon, since it exhibits greater or lesser deviation from the ideal crystal structure. It has small periodic regions, but exhibits diffuse x-ray reflections and is therefore referred to as x-ray-amorphous. Glass-like microporous carbon has a lower density at 1.2-1.6 g/cm³ than crystalline graphite at 2.2 g/cm³, and a narrow pore size distribution. Owing to the high proportion of open porosity, the carbon has excellent suitability as an adsorbent.

Amorphous or microcrystalline carbon films have high chemical inertness. They consist of sp² and sp³ bonds and therefore exhibit properties between those of graphite and diamond. The graphite-like structures consist of a turbostratically disordered layer sequence in which the microcrystals are joined by highly deformed and disorientated graphite layers.

Different theories have been established in the past about the exact bonding conditions in x-ray-amorphous carbon. Robertson and O'Reilly propagated a model in which amorphous carbon consists of sp² bond clusters which are networked with one another via sp³ bonds. Côté and Liu, in contrast, expressed the theory that amorphous carbon consists solely of sp² bonds which are three-dimensionally networked. Experiments supported the conjecture of dispersed crosslinked sp² bonds. In amorphous and nanocrystalline carbon, according to Lossy et al., a mixture of sp² and sp³ bond types is present. If these clusters become large enough, it is possible for nanocrystalline graphite and diamond structures to form.

In the case of use of nanoporous carbon as a membrane material, it is possible to achieve high permeances with simultaneously high selectivity. Gas permeation in carbon membranes is simpler than in polymer membranes since the gases do not dissolve in the carbon. A distinction is drawn between two kinds of carbon membranes: the molecular sieve carbon membranes (MSCM) and the adsorptive selective carbon membranes (ASCM). MSCMs allow the separation of gas mixtures with different molecule diameters due to the size exclusion of the larger molecule type.

In contrast, ASCMs allow the separation of gas mixtures with similar molecular radii or even the removal of larger from small molecules.

The permeation characteristics of adsorbable and nonadsorbable gases are based on different mechanisms. While the permeation of nonadsorbing gases in the ideal case can be understood as pure gas diffusion, the permeation of adsorbable gases is determined by surface diffusion processes. In mixtures of adsorbable and nonadsorbable gases, the permeation of the nonadsorbing component is hindered by the adsorbed gas molecules. The nonadsorbing gases first have to overcome a potential barrier for the diffusion through the membrane. In reality, gas diffusion and surface diffusion always occur at the same time. The separation works by the selective adsorption and surface diffusion of one component to the low-pressure side of the membrane, where it is ultimately desorbed.

Depending on the synthesis operation, such as sintering and mild activation operations, the pore sizes of the MSCMs are in the range between 2.8-5.2 Å. The pore sizes in ASCMs are slightly greater at 5-7 Å.

A further crucial advantage of carbon is the possibility of controlled adjustment of the pore size by thermochemical treatment, which enables production of membranes with different permeation and separation properties from the same starting material. Carbon membranes have a high chemical stability to acids, alkalis and organic solvents. Nanoporous carbon membranes, compared to polymer membranes, can be used under nonoxidizing conditions at high temperatures of up to 900° C.

One disadvantage of carbon lies in the limited stability thereof to oxidizing media and water vapor. Moreover, carbon membranes are more brittle than polymer membranes and can become blocked by adsorbing gases, for example chlorine gas. The adsorbed gas molecules can be removed again at temperatures of ≧200° C. The preferred process temperatures should therefore be within this range.

One of the first representatives of the group of glass-like carbons is “cellulose carbon” which forms as a result of pyrolytic conversion of cellulose to carbon under defined thermal treatment.

The production of carbon fiber membranes from cellulose fibers by pyrolysis is described, for example, in EP-A-0 671 202.

WO 00/62885 describes the use of polymeric phenolic resins for formation of carbon membranes suitable for gas separation. This involves dissolving the phenolic resins in methanol.

WO 01/97956 describes the production of nanoporous carbon membranes by pyrolysis of selected polymers on porous substrates. Preferred polymers mentioned include polyfurfuryl alcohol, polyvinyl chloride and polyacrylonitrile. These are mixed with additives, preferably titanium dioxide, silicon dioxide zeolites or polyethylene glycol, and applied to a substrate. This is followed by the pyrolysis.

EP-A-2 045 001 describes the use of phenol resins, melamine resins, urea resins, furan resins, polyimide resins or epoxy resins, and of resins based on polyethylene and cellulose, dissolved in organic solvents, for production of carbon membranes by film formation and subsequent pyrolysis of the resin.

Furthermore, EP-A-2 045 001 describes the construction of the carbon membrane, consisting of a porous substrate, subsequent porous ceramic layers, and the carbon layer, with the last interlayer, to which the carbon membrane is applied, being prepared from a TiO₂ sol and having a pore diameter of between 0.3 and 20 nm. Moreover, the membrane was tested only for the separation of water and ethanol by pervaporation at 75° C. This test does not allow suitability for gas separation at higher temperatures.

Before the pyrolysis step, the polymer membranes are often subjected to a physical or chemical pretreatment. Physical pretreatment is understood to mean, for example, the stretching of hollow fibers. The chemical treatment processes include, for example, treatment with carbonization catalysts such as mineral acids, or treatment with chemicals such as dimethylformamide to achieve narrow pore size distributions. A further pretreatment method is that of treatment under oxidizing atmosphere such as air at elevated temperature.

The pyrolysis of the synthetic polymers under inert gas atmosphere gives rise to high-porosity carbon, the pore sizes of which can vary greatly as a function of the type and morphology of the polymer and the pyrolysis conditions. Controlled heat treatment can adjust the pore size of the carbon membranes. The pyrolysis conditions have an important influence on the separation properties of the carbon. Heating rate, pyrolysis temperature and pyrolysis time are parameters adjustable in a defined manner. Supply of thermal energy breaks the bonds in the polymer, and the result is formation of the carbon membrane.

The known membranes do not yet have a suitable profile of properties for all separation tasks. In some cases, the membranes cannot be produced in a reproducible manner, i.e. the membrane properties vary in each batch.

It is an object of the present invention to provide a process for production of carbon membranes suitable for gas separation, and the resulting membranes should avoid the disadvantages of the prior art and should exhibit excellent separation properties with ideal reproducibility.

Outstanding separation properties for the separation of hydrogen from a gas mixture at temperatures between 150 to 450° C., as determined from mixture-gas measurements; are considered to be hydrogen permeances of ≧2 Nm³/m²/h/bar and hydrogen permselectivities ≧30.

-   Definition of permeance: flow rate in cubic meters under standard     temperature and pressure conditions (Nm³) of gas per m² of membrane     area, time and pressure, the pressure (driving force) used being the     partial pressure difference between feed/retentate and permeate of     the corresponding gas. -   Definition of permselectivity: the permselectivity is then given by     the ratio of the permeances ascertained.

The object is achieved by a process for producing carbon membranes suitable for gas separation, comprising the steps of:

a) coating a porous substrate with a solution of at least one organic polymer which can be converted to a carbon membrane by pyrolysis, b) drying the polymer coating on the porous substrate by removing the solvent, c) pyrolyzing the polymer coating on the porous substrate to form the carbon membrane suitable for gas separation, it being possible to conduct any of steps a) to c) or the sequence of steps a) to c) more than once, and the pyrolysis in step c) being effected at a temperature higher than the baking temperature of the porous substrate.

The object is also achieved by carbon membranes obtainable by the above process.

It has been found in accordance with the invention that particularly advantageous carbon membranes are obtained when the pyrolysis of the organic polymer is effected at a temperature higher than the baking temperature of the porous substrate.

The baking temperature of the porous substrate is understood to mean the baking temperature of the uppermost substrate layer to which the solution of the at least one organic polymer is applied. “Baking temperature” is understood to mean the temperature at which this uppermost layer of the substrate is converted to the form in which it is also present in the later carbon membrane.

Inorganic membranes generally have an asymmetric structure. Intermediate layers with decreasing pore size are applied to a coarse-pore carrier before the actual coating with the separation-active membrane layer based on the organic polymer. Only in this way is it generally possible to produce very thin and hence highly permeable membrane layers. The individual intermediate layers are baked in separate heat treatment steps, the temperature being lower from coating to coating. This is necessary in order to avoid shrinkage and alteration of the pore size and porosity of the layers below, which would lead to the formation of stresses and can result in defects.

In contrast to this it has been found that carbon membranes with outstanding separation properties are obtained if the pyrolysis temperature of the carbon membrane is higher than the baking temperature of the topmost substrate layer.

Preferably, the porous substrate has a surface mesoporous ceramic oxide layer atop a substrate carrier layer, said surface mesoporous ceramic oxide layer having a baking temperature correspondingly below the pyrolysis temperature of the organic polymer.

Preferably, this surface mesoporous ceramic oxide layer has a mean pore diameter in the range from 2 to 10 nm.

Preferably, the surface mesoporous ceramic oxide layer is produced by a sol-gel technique. Corresponding sol-gel processes are known to those skilled in the art.

The substrate carrier layer may be selected from any suitable materials. Preferably, it is selected from porous metals, porous ceramics, porous glasses or porous composites thereof.

The surface mesoporous ceramic oxide layer is preferably formed from TiO₂, ZrO₂, Al₂O₃, SiO₂ or mixtures thereof.

More preferably, the surface mesoporous ceramic oxide layer is formed from γ-Al₂O₃.

It is particularly advantageous when the surface mesoporous ceramic oxide layer in unbaked form is coated with the solution of the at least one organic polymer. Only in the subsequent pyrolysis is this surface mesoporous ceramic oxide layer then baked.

The pyrolysis is preferably performed at temperatures in the range from 500 to 900° C.

In the course of production of the inventive carbon membranes, due to the high mechanical and thermal stability thereof, particular preference is given to using porous ceramic carriers. The uppermost, narrowest-pore intermediate layer is a mesoporous oxide layer. According to the IUPAC definition, the pore size in a mesoporous layer is 2 to 50 nm. More preferably, the uppermost layer, the surface mesoporous ceramic oxide layer, has a mean pore diameter in the range from 2 to 10 nm.

A polymer solution is applied to this intermediate layer as a precursor. The polymer is dried, optionally crosslinked at slightly elevated temperature and then converted (pyrolyzed) to the carbon membrane by heat treatment or pyrolysis. The pyrolysis is more preferably performed at a temperature in the range from 600 to 800° C.

In the case of use of mesoporous oxide layers for the production of carbon membranes, it has been found in accordance with the invention that the separation properties of the membrane improve when the pyrolysis takes place at higher temperature than the baking temperature of the mesoporous oxide layer. The best results have been achieved when the baking of the mesoporous oxide layer before the pyrolysis has been dispensed with entirely.

The further properties of the porous substrate are explained in detail hereinafter.

According to the invention, any suitable organic polymers can be used as the solution which can be converted to a carbon membrane by pyrolysis. The person skilled in the art is aware of suitable polymers. Reference may be made, for example, to the literature cited at the outset.

Preferably, the at least one organic polymer is selected from cellulose, polymeric phenolic resins, melamine resins, urea resins, furan resins, polyimide resins, epoxy resins, polyfurfuryl alcohol, polyvinyl chloride, polyacrylonitrile, polyethylene, ethylenically unsaturated polyesters and mixtures thereof.

More preferably, in accordance with the invention, ethylenically unsaturated polyesters or mixtures thereof are used. In one embodiment of the invention, the organic polymer is not an ethylenically unsaturated polyester.

The ethylenically unsaturated polyesters used may be any suitable ethylenically unsaturated polyesters. The ethylenically unsaturated polyester preferably has repeat units of aliphatic diols, aromatic dicarboxylic acids and ethylenically unsaturated dicarboxylic acids. The ethylenically unsaturated polyester used in accordance with the invention can be prepared proceeding from aliphatic diols, aromatic dicarboxylic anhydrides and ethylenically unsaturated dicarboxylic anhydrides.

The aliphatic diols are preferably C₂₋₁₂-alkanediols, more preferably C₂₋₆-alkanediols, especially C₂₋₄-alkanediols. They may be linear or branched alkanediols in which the hydroxyl groups are terminal or nonterminal. Preference is given to using linear aliphatic alkanediols in which the hydroxyl groups are terminal or nonterminal. Especially preferably, 1,2-propanediol is used as the alkanediol.

The ethylenically unsaturated dicarboxylic acid is preferably maleic acid. It is especially used for preparation of the ethylenically unsaturated polyester in the form of maleic anhydride.

The aromatic dicarboxylic acids may be selected from any suitable aromatic dicarboxylic acids. They may, for example, be phthalic acid, isophthalic acid or terephthalic acid. The aromatic dicarboxylic acid is preferably phthalic acid, which is used for preparation of the ethylenically unsaturated polyesters in the form of phthalic anhydride.

In the ethylenically unsaturated polyester, diols and dicarboxylic acids may be present in equivalent amounts. Preference is given to working with a slight diol excess, such that the resulting polyesters have more alcohol end groups than carboxylic acid end groups. The excess of diol over dicarboxylic acid may be 1 to 20% by weight, preferably 2 to 10% by weight, especially 3 to 7% by weight.

Aromatic dicarboxylic acids and ethylenically unsaturated dicarboxylic acids can be used in any suitable molar ratios. Aromatic dicarboxylic acids and ethylenically unsaturated dicarboxylic acids are preferably used in a molar ratio of 0.1:1 to 1:0.1, more preferably 0.3:1 to 1:0.3, especially 0.09:1 to 1:0.9. Particular preference is given to working with equimolar amounts of aromatic dicarboxylic acids and ethylenically unsaturated dicarboxylic acids.

The monomers mentioned are used to prepare the ethylenically unsaturated polyesters under the customary conditions, and the ethylenically unsaturated groups of the ethylenically unsaturated dicarboxylic acids should be substantially preserved. For the preparation of the ethylenically unsaturated polyesters, reference may be made, for example, to known processes.

Ethylenically unsaturated polyesters used with preference in accordance with the invention have a molecular weight in the range from 100 to 2000 g/mol, more preferably 200 to 600 g/mol. The viscosity of the polyesters is preferably 4 to 200 mPas, more preferably 10 to 40 mPas.

The solution of ethylenically unsaturated polyesters preferably additionally comprises a crosslinking ethylenically unsaturated monomer and a free-radical former as a crosslinking initiator. The crosslinking ethylenically unsaturated monomer is preferably styrene or α-methylstyrene. The free-radical formers used may be any free-radical polymerization initiators. For example, it is possible to use dibenzoyl peroxide as the free-radical former. On application of the solution of ethylenically unsaturated polyesters to the porous substrate, the polyester is preferably as yet uncrosslinked or at most partially crosslinked to such an extent that its flowability is preserved.

Useful solvents are all solvents suitable for polyesters, preferably styrene, α-methylstyrene and acetone.

In one embodiment of the invention, the crosslinking ethylenically unsaturated monomer is used as a solvent, which means that, when the ethylenically unsaturated monomer is used, a further solvent is dispensed with. In the case of suitable adjustment of the viscosity of the ethylenically unsaturated polyester, it is likewise possible, in one embodiment of the invention, to dispense with the addition of a solvent. This is the case especially when the ethylenically unsaturated polyester is free-flowing at ambient temperature (22° C.).

More preferably, the ethylenically unsaturated polyester is used together with the crosslinking ethylenically unsaturated monomer without the addition of further solvents or diluents in the coating in step a).

In this case, the proportion of the crosslinking ethylenically unsaturated monomer in the mixture of ethylenically unsaturated polyester and ethylenically unsaturated monomer is preferably 0.1 to 5% by weight, more preferably 0.2 to 2% by weight, or preferably 20 to 90% by weight, more preferably 40 to 80% by weight.

In general, the solvent used for the organic polymers is an organic solvent, preferably styrene, α-methylstyrene and acetone.

The concentration of the organic polymer in the solution is preferably 20 to 95% by weight or 10 to 80% by weight, more preferably 40 to 70% by weight or 20 to 60% by weight.

The porous substrate can be coated with the solution of the polymers, preferably of the ethylenically unsaturated polyesters, by any suitable coating processes. The solution can be applied by dipping, spraying, impregnation processes, etc. Preference is given to using a dip coating process to apply the solution of ethylenically unsaturated polyester. The application of the ethylenically unsaturated polyester can also be achieved by ink coating and ultrasonic deposition (UD). Corresponding processes are described, for example, in EP-A-2 045 001 and WO 01/97956.

After the coating of the porous substrate with the solution of the polymer, for example of the ethylenically unsaturated polyester, the polyester coating is dried on the porous substrate. This removes any solvent present. In the case of use of a crosslinking ethylenically unsaturated monomer, excess amounts of this monomer are likewise removed in the course of drying. The drying is preferably conducted at a temperature in the range from 0 to 70° C., more preferably 17 to 30° C., especially at ambient temperature (22° C.), for a period of 0.1 to 50 hours, preferably 2 to 24 hours.

Before the pyrolysis in step c), after the drying b) or as part of the drying, curing can be conducted at slightly elevated temperatures. Curing is preferably effected at temperatures in the range from 20 to 150° C. or 50 to 250° C., more preferably 60 to 100° C., for a period of 1 to 100 hours, preferably 6 to 24 hours or 4.5 to 20 hours, especially 6 to 12 hours. The curing crosslinks the polymer, for example the ethylenically unsaturated polyester. The crosslinking prevents the melting and hence also the structural rearrangement of the polymer, for example of the crosslinked polyester, to the energetically more favorable state of graphite.

Before the pyrolysis step, the polymer membranes can also be subjected to a physical or chemical pretreatment. The chemical pretreatment processes include, for example, treatment with carbonization catalysts such as mineral acids, or treatment with chemicals such as dimethylformamide to achieve narrow pore size distributions. Additives such as polyethylene glycol, Lewis acids and ionic salts have been described in the literature cited at the outset, especially in EP-A-0 671 202. Such a pretreatment can also be effected in accordance with the invention, but is often unnecessary since a suitable pore size structure is obtained merely through use of the polyesters used in accordance with the invention. Further suitable pore formers are SO₃NH(C₂H₅)₃, triethylammonium nitrate and (—C(CF₃)₂—)hexafluorisopropylidene groups. The permeability of the membrane can be controlled via the type and amount of the pore formers.

At the time of pyrolysis, the polymers, preferably polyesters, used in accordance with the invention are preferably either already crosslinked or cure in the low-temperature range of the pyrolysis. Before the crosslinking, preferably all solvents should be removed from the polymer coating since they can lead to bubble formation and to defects in the polymer layer in the course of heating. The degree of crosslinking and crosslinking temperature can influence the permeability and selectivity of the later membrane.

The pyrolysis of the polymer coating, preferably polyester coating, on the porous substrate to form the carbon membrane in step c) is preferably performed under inert gas at temperatures in the range from 500 to 900° C., more preferably from 650 to 750° C. The pyrolysis is preferably performed for a period of 0.1 to 24 hours, preferably 0.5 to 17 hours more preferably 1 to 10 hours, in particular 1 to 2 hours.

Under some circumstances, it may be advisable to perform the pyrolysis in the presence of small amounts of oxygenous gases. However, preference is given in accordance with the invention to performing the pyrolysis under inert gas, for example under a nitrogen and/or argon atmosphere.

The pyrolysis gives rise to high-porosity carbon, the pore sizes of which can be varied as a function of the type and morphology of the precursor and the pyrolysis conditions. Controlled heat treatment can adjust the pore size of the carbon membrane. The pyrolysis conditions have an influence on the separation properties of the resulting carbon membrane. The heating rate, pyrolysis temperature and pyrolysis time are adjustable parameters. Supply of thermal energy breaks the bonds in the polymer. During the decomposition of the polymer, a series of gaseous by-products is formed, as a result of which a significant loss of mass occurs.

After the pyrolysis, the resulting carbon membrane, if required and desired, can be subjected to further treatment steps. For example, the membrane can be activated by heat treatment under air at temperatures of 100° C. to 500° C. Activation in a carbon dioxide stream is also possible in accordance with the invention. However, preference is given in accordance with the invention to not performing any such aftertreatment after the pyrolysis in step c), and the carbon membranes which result from the pyrolysis are instead directly suitable for gas separation.

The carbon membrane obtained in accordance with the invention preferably has a mean pore diameter in the range from 0.1 to 0.7 nm, more preferably 0.25 to 0.45 nm or to 0.4 nm, especially 0.3 to 0.4 nm. These membranes preferably allow only molecules with a relatively small kinetic gas diameter, which can penetrate into the pore system of the membrane, to pass through. These are especially hydrogen, water, ammonia and helium. The mechanism of the separation of these gases is not based on different adsorption and transport characteristics of the gases to be separated, but on shape-selective separation (molecular sieving). In this case, it is ensured that the flow of the permeating gases rises in a linear manner with the driving force.

Particular preference is given to effecting the following separations by means of the carbon membrane obtained in accordance with the invention, optionally in combination with chemical reactions to shift the equilibrium:

separation of hydrogen from hydrocarbons from C₁ upwards, for example in dehydrogenations.

According to the invention, the target use temperatures are in the range from 200 to 500° C.

The inventive carbon membranes are preferably produced in the form of asymmetric membranes. The selection of the porous substrate (carrier) depends firstly on the chemical reactivity and compatibility with the carbon, the mechanical stability, but also on economic factors such as costs and availability. Suitable porous substrates are, as specified above, porous materials such as porous metals, porous ceramics, porous glasses or porous composites thereof.

The porous substrate more preferably has a mesoporous ceramic oxide layer of TiO₂, ZrO₂, Al₂O₃, SiO₂ or mixtures thereof, which is coated with the solution of the polymer, preferably of the ethylenically unsaturated polyester.

The mesoporous ceramic oxide layer is more preferably formed from γ-Al₂O₃.

The mesoporous ceramic oxide layer has a baking temperature below the pyrolysis temperature in step c). This mesoporous ceramic oxide layer is preferably coated in unbaked form with the solution of the polymer, preferably of the ethylenically unsaturated polyester, in which case the baking of the mesoporous ceramic oxide layer is simultaneous with the pyrolysis. This can lead to a particularly advantageous spectrum of properties of the resulting carbon membrane.

The use of steel as the carrier material gives rise to the advantage of standardized, well-controllable and reproducible manufacture. In the case of use of steel, it is, however, impossible to produce porous carriers from a layer composite, which would allow high permeabilities and selectivities to be combined with one another.

Therefore, carbon membranes on ceramic porous carriers are preferred. They preferably consist of a composite of ever finer layer thicknesses and pore sizes. Since defects in the surface of the substrates can cause defects in the carbon membrane, such as pinholes, which can lead to the loss of the molecular sieving properties of the membrane, it is advantageous to reduce the surface topography and the defect structure of the carrier by coating the porous carrier with intermediate layers.

The basic structure of the preferred inventive membrane is described in EP-A-2 045 001. Reference may be made especially to FIG. 1 therein with the corresponding explanations in the description.

On the preferred coarse-pore Al₂O₃ carrier, a smaller-pore layer is preferably followed by a mesoporous ceramic oxide layer of γ-Al₂O₃ with a pore diameter in the range from preferably 2 to 10 nm, more preferably 3 to 7 nm, especially preferably 4 to 6 nm. The layer thickness of this mesoporous ceramic oxide layer is preferably 0.1 to 10 μm, more preferably 0.5 to 5 μm, especially 1 to 2 μm, in particular 1 to 1.5 μm.

By virtue of the fact that the pore diameter in the porous substrate cross section decreases toward the carbon membrane, a pressure drop in the porous substrate can be avoided, while at the same time establishing suitable pore sizes in the carbon membrane. This design of the membrane can ensure high flows through the membrane in the particular separation task.

The invention is illustrated in detail by the examples below.

EXAMPLES Example 1

The precursor solution was prepared by reaction of a diol with at least one unsaturated carboxylic acid to give an unsaturated polyester and subsequent addition of an olefin. In the specific case, in a 250 ml two-neck flask, 19.98 g of 1,2-propanediol (0.25 mol+5% excess; VWR), 12.26 g of maleic anhydride (0.125 mol; VWR) and 18.52 g of phthalic anhydride (0.125 mol; VWR) were weighed in. Subsequently, 0.01 g of hydroquinone (VWR) was added as a polymerization inhibitor, in order to prevent premature polymerization or crosslinking processes. Under nitrogen, the raw materials weighed in were heated to a reaction temperature of 200° C. by means of an oil bath. To check chain formation, the acid number was determined by titration with KOH solution at regular intervals. At an acid number of approx. 50, the reaction was stopped by rapid cooling to 140° C. With the aid of a dropping funnel, 50 g of styrene were subsequently added as a crosslinker within one minute while stirring vigorously. The styrene added had previously been heated to 50° C. In order to prevent premature polymerization of the polyester-styrene mixture, the unsaturated polyester resin solution was cooled to room temperature in a water bath. Thereafter, 1% dibenzoyl peroxide (VWR) as a free-radical former was added to the polyester-styrene mixture and then the mixture was stirred at room temperature for 5 minutes.

The carriers used for the membranes were ceramic single-channel tubes of α-Al₂O₃ of length 105 mm with internal diameter 7 mm and external diameter 10 mm, which had intermediate layers with decreasing pore size on the inside. The uppermost intermediate layer used was a boehmite sol, which was dried at room temperature and sintered at 300° C., 500° C. and also 650° C.

This was coated in a dip coating process with the clear, viscous, pale yellowish precursor solution. For this purpose, the solution was introduced into the tube and, after a residence time of 1 minute, pumped out again. The sample was dried under air for 24 h and then cured in a drying cabinet at a temperature of 80° C. for 12 h in m/A and cooled to room temperature. After the crosslinking of the polymer to give a thermoset, the layer was pyrolyzed to carbon by the following procedure:

-   1. increase in temperature at 0.5 K/min from room temperature to     380° C. under nitrogen -   2. holding time of 1 hour at 380° C. under nitrogen -   3. increase in temperature at 0.6 K/min from 380 to 500° C. under     nitrogen -   4. holding time of 10 minutes at 500° C. under nitrogen -   5. switch of atmosphere from nitrogen to argon -   6. increase in temperature at 1 K/min from 500° C. to the final     pyrolysis temperature under argon -   7. holding time of 1 hour at the final pyrolysis temperature under     argon -   8. cooling at 5 k/min from the final pyrolysis temperature to     500° C. under argon -   9. switch of atmosphere from argon to nitrogen -   10. cooling at 5 K/min from 500° C. to room temperature under     nitrogen

The composition of the polyester, the crosslinker fraction and the final pyrolysis temperatures are given in the results table below. The topmost interlayer was converted in this procedure into a γ-Al₂O₃ layer having a pore size of 5 to 9 nm and a thickness of 1 to 2 μm.

Example 2

A boehmite sol is applied by means of dip-coating to the inside of a tubular asymmetric ceramic carrier, consisting of a composite of several layers with decreasing pore size. The carriers have an internal diameter of 7 mm, an external diameter of 10 mm and a length of 105 mm. After the coating, the sample is dried under air at room temperature and sintered at 300° C., 500° C. and 800° C. The dried and sintered boehmite sol layer was coated with the precursor by means of dip-coating. For that purpose, the phenolic resin solution was poured into the membrane tube, and drained off again after a residence time of 1 minute. The solution was prepared using two phenolic resin powders:

Company Type Mean molar weight Hexamine fraction FERS FB8001 2685 g/mol 8.9% HEXION 0235DP  900 g/mol —

The solution was prepared by first dissolving 7.5 g of phenolic resin powder in 22.5 g of methanol (MERCK). Then 20 g of 1-methyl-2-pyrrolidone (≧99.5%, MERCK) were added as further solvent. After the coating, the polymer film is dried under air at room temperature for 24 hours to fully remove the solvent. After the drying, the polymer is crosslinked in a drying cabinet at temperatures of 150° C. over a period of 3 hours to give a thermoset, and cooled to room temperature. The layer was then pyrolyzed to carbon, using the following procedure:

-   1. increase in temperature at 0.5 K/min from room temperature to     380° C. under nitrogen -   2. holding time of 1 hour at 380° C. under nitrogen -   3. increase in temperature at 0.6 K/min from 380 to 500° C. under     nitrogen -   4. holding time of 10 minutes at 500° C. under nitrogen -   5. switch of atmosphere from nitrogen to argon -   6. increase in temperature at 1 K/min from 500° C. to the final     pyrolysis temperature under argon -   7. holding time of 1 hour at the final pyrolysis temperature under     argon -   8. cooling at 5 k/min from the final pyrolysis temperature to     500° C. under argon -   9. switch of atmosphere from argon to nitrogen -   10. cooling at 5 K/min from 500° C. to room temperature under     nitrogen

The type of phenolic resin used and the final pyrolysis temperature are given in results table 1 below. In the pyrolysis step, the polymer on the unsintered ceramic carrier membrane is finally decomposed to carbon at a heating rate of 0.5-1 K/min at temperatures between 600° C. and 900° C. (in particular 800° C.) under an inert gas atmosphere (nitrogen and/or argon).

In order to characterize the separation properties of the carbon membranes produced in examples 1 and 2, individual-gas and mixture-gas measurements were performed on a gas measurement setup, at a temperature of 150° C.

The permeation measurement of the individual gases was carried out using the pressure increase method, the permeance here being ascertainable from the linear pressure increase over time.

The mixture-gas measurements were carried out using an H₂/C₃H₈ mixture consisting of 80% by volume hydrogen and 20% by volume propane, with a transmembrane pressure of 1.5 bar (feed pressure=2.5, permeate=1.0 bar absolute). The total flow rate was measured by means of gas bubble counter, and the permeances were calculated with the aid of the feed pressure and permeate pressure for H₂ and propane from the composition of the feed, the retentate, and the permeate. The permeate indicates the flow rate per m² membrane surface, time and pressure, the pressure used being the partial pressure difference (driving force) of the gas in question. The permselectivity is then given by the ratio of the two permeances ascertained.

TABLE 1 Baking Dicarboxylic Individual Mixture measurement temperature acid Crosslinker Crosslinking Pyrolysis measurements Permeance Perm- of support (Mol (Mol Styrene Tem- Tem- Permeance (Nm³/m²/ selec- Mem- membrane %) %) (% by perature Time perature Time (Nm³/m²/h/bar)* h/bar)* tivity brane (° C.) Precursor Ma Pa weight) (° C.) (h) (° C.) (h) H2 C3 SF6 H2 H2/C3 PES-1 unsintered PES 25 25 50 80 12.0 650 1.0 10.2 0.062 0.030 6.9 41 PES-2 300 PES 25 25 50 80 12.0 700 1.0 4.7 0.007 4.0 362 PES-3 500 PES 25 25 50 80 12.0 650 1.0 6.8 0.006 0.002 5.6 418 PES-4 500 PES 25 25 50 80 12.0 700 1.0 5.7 0.002 4.8 68884 PES-5 500 PES 7 43 50 80 12.0 700 1.0 4.6 0.069 0.034 3.1 19 PES-6 500 PES 40 10 50 80 12.0 700 1.0 3.1 0.000 0.000 2.5 29703 PES-7 500 PES 25 25 50 80 4.5 750 1.0 2.3 0.001 0.000 3.1 767 PES-8 500 PES 25 25 50 80 12.0 700 1.0 6.2 0.002 0.001 4.2 594 PES-9 500 PES 25 25 50 250 1.0 750 1.0 7.6 0.006 0.003 PH-1 unsintered PH (FB8001) — — — 150 3.0 800 1.0 10.24 0.258 0.322 7.4 25 PH-2 300 PH (FB8001) — — — 150 3.0 800 1.0 5.32 0.240 1.054 2.2 5 PH-3 400 PH (FB8001) — — — 150 3.0 800 1.0 5.30 0.029 0.036 3.0 29 PH-4 500 PH (FB8001) — — — 150 3.0 800 1.0 6.99 0.082 0.011 5.9 103 PH-5 700 PH (FB8001) — — — 150 3.0 800 1.0 2.51 0.003 0.018 1.6 84 PH-6 900 PH (FB8001) — — — 150 3.0 800 1.0 3.44 0.004 0.018 2.5 124 PES = polyester PH (type) = phenolic resin Ma = maleic anhydride Pa = phthalic anhydride *Nm³ = m² at stp

Example 3

Preparation of the precursor solution (polyester: 25 mol % maleic anhydride, 25 mol % phthalic anhydride and 50 mol % 1,2-propanediol; solution: 50% by weight PES and 50% by weight styrene), the support used for the coating (topmost interlayer: boehmite sol sintered at 500° C.), the coating with the precursor solution, the drying (24 h at room temperature under air), the crosslinking (12 h at 80° C. under air), and the final pyrolysis (1 h at 650° C. under Ar) took place as described in detail in example 1.

To investigate the separation properties of the carbon membranes, mixture-gas measurements were carried out on a test setup at temperatures between 150 and 450° C. and also at feed pressures between 1 and 10 bar absolute. The measurements were made using a H₂/C₃H₈ mixture consisting of about 50% by volume hydrogen and 50% by volume propane; the permeate pressure was approximately 1 bar absolute. The overall retentate flow rate and overall permeate flow rate were measured using gas meters, and the gas composition of the feed, retentate and permeate by means of an online gas chromatograph. In addition, the pressures and temperatures were recorded. The individual flow rates (units=Nm³/h/m²; Nm³=cubic meter under standard temperature and pressure conditions (STP)) were calculated from the overall flow rate and from the composition of the permeate. The permeances (flow rate standardized to the driving force, units=Nm³/h/m²/bar, driving force=partial pressure difference between feed/retentate and permeate) are then given by the compositions of the feed/retentate and of the permeate, and also the associated pressures. The permselectivity is given by the ratio between the two permeances ascertained.

Table 2 shows the data measured and calculated for the membrane under different conditions:

-   I: Variation of the feed/retentate pressure between 2.6 and 10.2 bar     absolute at 300° C. -   II: Variation of the temperature between 200 and 350° C. at a     feed/retentate pressure of 7.1 bar absolute.

The results show that, at a constant temperature of 300° C., with an increase in the transmembrane pressure (TMP=arithmetic mean of feed pressure and retentate pressure minus permeate pressure), the hydrogen permeance drops from 9.9 to 5.9 Nm³/h/m²/bar and the permselectivity from 930 to 400.

The temperature increase with a constant feed/retentate pressure of 7.1 bar absolute shows that not only the flow rate and the permeance of hydrogen but also the hydrogen-to-propane permselectivity are increased.

TABLE 2 Permeate Perm- Feed/retentate Flow Permeance P selec- Temper- Retentate Concentration rate Concentration Flow Flow C3 H2 tivity ature flow rate Pressure C3 H2 Pressure (Nm³/h/ C3 H2 rate C3 rate H2 TMP (Nm³/h/ P(H2)/ (° C.) (Nm³/h) (bar) (vol %) (vol %) (bar) m²) (vol %) (vol %) (Nm³/h/m²) (Nm³/h/m²) (bar) m²/bar) P(C3) I: Variation of pressure 300 0.18 2.6 50.4 49.6 1.1 1.9 0.7 99.3 0.014 1.9 1.5 0.011 9.9 932 300 0.18 4.2 51.8 48.2 1.1 6.6 0.4 99.6 0.026 6.6 3.0 0.012 7.6 637 300 0.18 7.2 54.4 45.6 1.2 14.2 0.4 99.6 0.052 14.1 6.0 0.013 6.6 495 300 0.18 10.2 51.0 49.0 1.2 22.5 0.3 99.7 0.076 22.4 9.0 0.015 5.9 400 II: Variation of temperature 200 0.17 7.1 51.4 48.6 1.1 11.5 0.4 99.6 0.046 11.4 5.9 0.013 5.0 394 250 0.17 7.1 51.8 48.2 1.2 12.7 0.4 99.6 0.045 12.7 5.9 0.012 5.6 457 300 0.18 7.2 54.4 45.6 1.2 14.2 0.4 99.6 0.052 14.1 6.0 0.013 6.6 495 350 0.17 7.1 52.3 47.7 1.2 15.8 0.3 99.7 0.052 15.8 5.9 0.014 7.1 505 

1. A process for producing carbon membranes suitable for gas separation, comprising the steps of: a) coating a porous substrate with a solution of at least one organic polymer which can be converted to a carbon membrane by pyrolysis, b) drying the polymer coating on the porous substrate by removing the solvent, c) pyrolyzing the polymer coating on the porous substrate to form the carbon membrane suitable for gas separation, it being possible to conduct any of steps a) to c) or the sequence of steps a) to c) more than once, and the pyrolysis in step c) being effected at a temperature higher than the baking temperature of the porous substrate.
 2. The process according to claim 1, wherein the porous substrate has a surface mesoporous ceramic oxide layer atop a substrate carrier layer, said surface mesoporous ceramic oxide layer having a baking temperature below the pyrolysis temperature of the organic polymer.
 3. The process according to claim 2, wherein the surface mesoporous ceramic oxide layer has a mean pore diameter in the range from 2 to 10 nm.
 4. The process according to claim 2, wherein the substrate carrier layer is selected from porous metals, porous ceramics, porous glasses and porous composites thereof.
 5. The process according to claim 2, wherein the surface mesoporous ceramic oxide layer has been produced by a sol-gel technique.
 6. The process according to claim 2, wherein the surface mesoporous ceramic oxide layer is formed from TiO₂, ZrO₂, Al₂O₃, SiO₂ or mixtures thereof.
 7. The process according to claim 6, wherein the surface mesoporous ceramic oxide layer has been formed from γ-Al₂O₃.
 8. The process according to claim 1, wherein the surface mesoporous ceramic oxide layer in unbaked form is coated with the solution of the at least one organic polymer.
 9. The process according to claim 1, wherein the pyrolysis in step c) is performed at temperatures in the range from 500 to 900° C., preferably at temperatures up to 500° C. under nitrogen and above 500° C. under argon.
 10. The process according to claim 1, wherein the at least one organic polymer is selected from cellulose, polymeric phenolic resins, melamine resins, urea resins, furan resins, polyimide resins, epoxy resins, polyfurfuryl alcohol, polyvinyl chloride, polyacrylonitrile, polyethylene, ethylenically unsaturated polyesters and mixtures thereof.
 11. The process according to claim 10, wherein the ethylenically unsaturated polyester has repeat units of aliphatic diols, aromatic dicarboxylic acids and ethylenically unsaturated dicarboxylic acids.
 12. The process according to claim 11, wherein the aliphatic diols are C₂₋₁₂-alkanediols and the ethylenically unsaturated dicarboxylic acid is maleic acid.
 13. The process according to claim 10, wherein the solution of ethylenically unsaturated polyesters additionally comprises a crosslinking ethylenically unsaturated monomer and a free-radical former as a crosslinking initiator.
 14. The process according to claim 13, wherein the crosslinking ethylenically unsaturated monomer is styrene or α-methylstyrene.
 15. A carbon membrane obtainable by a process according to claim
 1. 